Science News
04-27-07
Gardeners' Friend Causes a Stink
Janet Raloff
Certain insects offer gardeners and commercial growers a green alternative to
synthetic pesticides. High on the list of these beneficial agents are blackspotted bugs, known alternately as ladybugs, ladybirds, and lady beetles.
Whatever they're called, the insects' appetite for aphids makes them a
gardener's friend.
Among vintners, however, an Asian member of the ladybug family now rapidly
spreading throughout the United States has acquired quite a different
reputation: They're stink bugs.
When crushed or even just stressed, the insects release blood from their leg
sockets. Several volatile compounds in that blood are highly aromatic, with
distinctly vegetal scents, notes Iowa State University analytical chemist
Lingshuang Cai. The barest whiff can bring to mind a mix of green bell
peppers, roasted peanuts, raw potatoes, and musty earth. It may not sound too
bad, she says, but "it's actually quite noxious" and can definitely spoil a batch
of wine. Indeed, her team notes, "entire harvests and some vineyards have
failed because of multicolored Asian ladybug contamination."
Why wine? Because these pretty little ladybugs have a penchant for noshing
on damaged grapes and so can easily be harvested along with the fruit and
crushed during winemaking.
At the American Chemical Society's spring meeting in Chicago last month, Cai
reported that her team had confirmed the role of several specific chemicals in
the Asian ladybugs' distasteful odor, one of which was a surprise. Her team's
research also showed that among the tested bugs, orange Asian ladybugs
produced dramatically more of the smelly compounds than their yellow-coated
kin—even though both varieties belong to the same species (Harmonia
axyridis).
Like Sauvignons . . . but more so
Since 1916, U.S. farmers have periodically tried introducing this beetle, in the
hope it might vanquish the aphids that attack plants such as soybeans. Early
efforts failed, and the beetles—sometimes called harlequin ladybugs for their
variable coloring—disappeared. However, efforts in the 1970s and '80s
appeared to take, and by 1988, the insect was firmly ensconced in Louisiana.
The harlequin is now the most widespread ladybug species in North America.
The bug also recently spread—not by invitation—to Europe.
Noxious ladybug smells trace to compounds known as methoxypyrazines.
However, Australian researchers 15 years ago identified some members of this
chemical family as contributing to the distinctive aroma of Sauvignon blanc
wines. Scents emitted by Cabernet Sauvignon and Cabernet franc wines—at
least those made from grapes grown in the United States—also owe some of
their vegetal notes to methoxypyrazines.
The bugs, however, can easily imbue wines with a broader mix and higher
concentrations of these chemicals, Cai says.
Although the Iowa State team's interest in studying the compounds was
motivated by the trouble they can cause vintners, the researchers didn't study
tainted wine. Instead, they extracted methoxypyrazines from the beetles.
Cai's group put highly absorbent fibers along with five ladybugs in each of
several vials. A day later, they removed the insects and analyzed chemicals
among the fibers. They identified 38 distinct compounds including four
methoxypyrazines: ones known as IPMP, SBMP, IBMP, and DMMP.
The first three chemicals on that list had been linked to wines before,
especially those tainted by ladybugs. However, Cai says, previous tests had
been unable to trace the methoxypyrazines' source or determine the extent to
which bugs contributed to a wine's off aroma or flavor. Her team's new focus
on the chemicals in ladybugs themselves "provides conclusive evidence" of the
insects' release of methoxypyrazines, she says. The last one on the list,
DMMP, was never before associated with wines or the beetles.
To be sure that the four compounds are the ones responsible for a ladybug's
foul smell, the researchers trained colleagues to recognize the insects' scent.
Later, these individuals sniffed each of the purified methoxypyrazines extracted
from the absorbent fibers. The tests confirmed that all four chemicals
contributed to the noxious smell of ladybugs and presumably ladybugcontaminated wines. Further, the testers agreed that IPMP, SBMP, and IBMP
smell like bell peppers, whereas DMMP had a largely musty smell.
Cai's group also quantified the release of each compound by the bugs, which
she surmises to be a defensive action. Concentrations of IPMP turned out to
be highest, and those of IBMP were lowest.
The harlequin
The harlequin species that the Iowa researchers studied exhibits wide
individual variation in shell hue. In the United States, the species ranges from
tomato red to yellow. In Asia, some H. axyridis specimens even have black
shells with red spots.
Cai told Science News Online that the biggest surprise in her research was
stumbling over a relationship between a bug's color and the output of three of
its methoxypyrazines. An average orange bug released 0.31 nanogram of
IPMP, 0.11 ng SBMP, and 0.60 picogram of IBMP. In contrast, yellow beetles
produced less than a tenth as much of these methoxypyrazines, the
researchers report in the April 13 Journal of Chromatography A. The scientists
didn't report such numbers for DMMP.
If color proves a solid determinant of the species' methoxypyrazine output, the
Iowa State researchers say, this could explain the discrepancy between their
new data and reports of far smaller methoxypyrazine concentrations
associated with the beetles. The color of a harlequin ladybug is determined in
part by genetics, but also to some extent by larval diet.
Paying attention to the color of ladybugs in vineyards might also give growers a
clue to how seriously their wines might be tainted with methoxypyrazines, Cai
says.
Gary Pickering and his colleagues at Brock University in Canada have been
investigating ways to remove the taint of methoxypyrazines in wines. Last year,
they reported some success by incubating red and white wines with oak chips.
The treatment "reduced the intensity" of the taint, they reported in the January
2006 International Journal of Food Science & Technology. The scientists tried
a host of other techniques, including irradiation of wine with ultraviolet light and
mixing in absorbent clay or activated charcoal, but none had more than a
"limited effect" on red wine and all had no effect on white wine.
For now, the only cure for ladybug-tainted wines is a quick trip down the drain
or perhaps nose plugs.